Officials: Ukraine's opposition being investigated for allegedly attempting to seize power

Pro-European Union activists gather during a rally in the Independence Square in Kiev, Ukraine, Sunday, Dec. 8, 2013. Over 200,000 angry Ukrainians occupied a central Kiev square on Sunday, to denounce President Viktor Yanukovych’s decision to turn away from Europe and align this ex-Soviet republic with Russia, as massive protests continued for a third week. (AP Photo/Alexander Zemlianichenko)
(The Associated Press)

Pro-European Union activists in military helmets march along streets to block government offices in Kiev, Ukraine, Sunday, Dec. 8, 2013. The third week of protests continue Sunday with an estimated 200,000 Ukrainians occupying central Kiev to denounce President Viktor Yanukovych’s decision to turn away from Europe and align this ex-Soviet republic with Russia. (AP Photo/Efrem Lukatsky)
(The Associated Press)

A man dressed as a Cossack on a horse parades through the street during a Pro-European Union rally in Independence Square, Kiev, Ukraine, Sunday, Dec. 8, 2013. The third week of protests continue Sunday with an estimated 200,000 Ukrainians occupying central Kiev to denounce President Viktor Yanukovych’s decision to turn away from Europe and align this ex-Soviet republic with Russia. (AP Photo/Efrem Lukatsky)
(The Associated Press)

KIEV, Ukraine – Ukraine's security service says it is investigating several opposition leaders suspected of attempting to seize power amid the massive anti-government protests gripping the country.

Sunday' statement by Ukraine's Security Service raised the stakes in the opposition's standoff with President Viktor Yanukovych.

The announcement came hours after hundreds of thousands of angry Ukrainians occupied the capital's main square and blockaded approaches to key government buildings during the third straight week of such protests.

The opposition responded by saying that any possible attempts to impose a state of emergency would lead to even more protests.

Sunday's protest — the biggest in Ukraine since its pro-democracy Orange Revolution in 2004 — denounced Yanukovych's recent decision to turn this ex-Soviet republic away way from Europe and closer to Moscow.